$300k each sounds high. I don't believe they had reports. It was its own fiefdom. Probably closer to ~$200k. I don't believe they are Bay Area–based, either, where 300 might make more sense.
A stripe software engineer in the US is easily making $300k, including equity, unless they’re a new grad. Add another 10-15% for their benefits and total cost to Stripe.
It seemed like a standard acquisition / acqui-hire. Buy out the business, Stripe owns it, the employees are paid like Stripe employees. Access to codebase is tangential, big tech co codebases are already very segmented off into tons of repo's with different access permissions. Ultimately, this is just conjecture.